1. Field of the Invention
This invention relates to a antenna patterns for a motor vehicle and, more particularly, a antenna patterns for a motor vehicle, wherein reception sensitivity of an AM broadcast wave is improved.
2. Description of the Prior Art
There is conventionally known a window glass antenna for a motor vehicle using, as radio reception antenna wires, a plurality of defogging heater wires additionally formed on the inner surface of a rear window of the motor vehicle.
The reception sensitivity of the AM (medium frequency band) antenna patterns is determined by the area of glass. That is, the conductor pattern area of the window glass antenna is determined by the glass area. The glass area is almost determined by an outline of the motor vehicle. Therefore, a motor vehicle having a small glass area has low reception sensitivity .
According to prior art techniques, a preamplifier is arranged between a car radio and the window glass antenna to amplify a reception output, or an impedance converter is arranged to establish matching, thereby reducing a transmission loss.
FIG. 7 shows such a conventional arrangement including a preamplifier. Electrodes 72 to 74 are formed on a rear window glass 71. Heater wires 75 are connected between the electrodes 72 and 73 and between the electrodes 72 and 74. The electrode 73 is grounded to the vehicle body and is connected to the ground input of a preamplifier 79. A heating current is supplied to the electrode 74 which is connected to the input terminal of the preamplifier 79.
The preamplifier 79 increases a reception output from the electrode 74 to a predetermined level, the output of which is supplied to a car radio 77 through a coaxial cable 76 whose outer braided conductor is grounded. Reference numeral 78 denotes a loudspeaker connected to the car radio 77.
FIG. 8 shows a system arrangement of another conventional example having an impedance converter. The same reference numerals as in FIG. 7 denote the same parts in FIG. 8. Referring to FIG. 8, a reception output from an electrode 74 is supplied to a car radio 77 through an impedance converter 80 and a coaxial cable 76.
Heater wires 75 exhibit a high impedance state in the AM broadcast wave band. The impedance converter 80 converts an impedance of the heater wires 75 serving as an antenna to match with that of the coaxial cable 76. By this impedance matching, reception outputs from the electrodes 73 and 74 are supplied to the car radio 77 with a low transmission loss.
In the above prior art, preamplifier or impedance converter necessarily increases the components of a window antenna system of a motor vehicle. When the window glass antenna for the motor vehicle is placed in a strong electric field, the reception signal is distorted due to saturation of the preamplifier. In addition, even if the impedance converter is arranged, a motor vehicle having a small glass area cannot have a sufficient reception sensitivity.